Marking plastic materials for various purposes is a general problem which has received various solutions that are satisfactory in special cases. However, as soon as it is necessary for the marking to withstand severe conditions of length of life, in particular in the face of abrasion, few solutions remain acceptable. If further constraints are added concerning the accuracy of marking definition, the retention of such accuracy over a long period of time, and the possibility of reading the marks from a distance and after the plastic material has been exposed to an unfavorable, e.g. abrasive, medium for several years, then none of the known prior solutions is satisfactory.
Accuracy and resistance to being removed are characteristics which are desirable in marks not only from the standpoint of general appearance. Accuracy and resistance become essential for marks constituting bar codes whose sole purpose is to be readable by optical reader devices. Such automatic reading by a device is only possible if the marking conserves adequate contrast over a period of time and it the bars of various pre-defined widths are drawn with sufficient accuracy, for example when bar width remains within 10% of nominal bar width, thereby requiring an absolute accuracy of about 1/100 mm.
Naturally, the marking method must also be rapid and cheap. The most effective prior art method at meeting these conditions is ink jet marking. However, it is not really satisfactory for marking plastic materials because the ink does not adhere adequately. Attempts have been made to remedy this by using a chemically aggressive ink which etches its way into the plastic material. Unfortunately, this method of marking cannot be used for marking bar codes capable of being read by optical reader devices and capable of surviving for a period of several years. This is because the ink migrates into the plastic material and the initial line width accuracy is not retained, thereby giving rise to a high percentage of read errors during automatic reading by reader devices.
It is also known that lasers are capable of being used for highly accurate work, but that most plastic materials are transparent to laser radiation and are thus insensitive to laser action.
Preferred implementations of the present invention provide a method capable of rendering plastic materials which are initially transparent to laser radiation sensitive to said radiation, thereby enabling them to be subsequently marked in an accurate and indelible manner by an operation which is both rapid and cheap.
However, in most circumstances merely making plastic materials sensitive to laser action does not suffice. Many plastic materials have a composition which has been precisely adapted to specific need, after long periods of testing and development. For example, ear tags have been used for a long time to identify animals, in particular cattle. These tags were initially made of metal, which was not very satisfactory because metal is too hard and too rigid and gives rise to injuries to the animal. Completely satisfactory ear tags can now be made out of plastic material, with the tabs being in the form of a plate which is 1.20 mm thick. Such a plate is flexible; it must also be mechanically strong. For cattle, it has become accepted practice for the plastic to have a hardness of 60 on the Shore scale, however, it has been observed that sheep need increased hardness of 74 on the Shore scale, although this is too hard for cattle. In addition, such a plate must not be subject to bacteriological attack. It must also withstand abrasion since a cow may wear it for an average of 8 years. Naturally, the plastic material which makes up the plate must be capable of being marked, and heretofore this has only been possible by depositing ink by a printing or a jet process. The ink marking, whether it is printed or projected, must also withstand wear. There is a French standard (number NF-T-54006) which defines the abrasion tests which the marking must be capable of withstanding. It is desirable for it to withstand 1,000 friction cycles. Ink marking has difficulty in satisfying this condition. However, for the reasons already mentioned above, ink marking is incapable of allowing bar codes to be used for identifying the animals.
The plastic material which satisfies the above-specified requirements and which is preferably adequate in practice is a polyurethane having a protective additive against bacteriological attack added thereto.
This material is completely transparent to laser radiation.
It is clear that making it sensitive to laser radiation must not cause it to lose its other qualities: in other words it is essential to avoid changing any of its properties which ensure that it is well adapted to its intended use.
Preferred implementations of the invention thus provide a method capable of making a plastic material or a plastic material based on composition sensitive to laser radiation without spoiling its other qualities.